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United States Patent |
5,719,307
|
Lassila
,   et al.
|
February 17, 1998
|
Diamine chain extenders and method of use
Abstract
Polyurethane-urea elastomers are made using as a chain extender
2-methyl-1,3-propanediol-bis-p-aminobenzoate which is the reduction
product from hydrogenating 2-methyl-1,3-propanediol-bis-p-nitrobenzoate.
The latter composition is preferably made by esterifying p-nitrobenzoic
acid and 2-methyl-1,3-propanediol using a stoichiometric excess of the
diol initially, adequate to render the reaction mixture processible, and
after converting to a nonvolatile form sufficient diol to form diester
with substantially all of the acid, removing free diol by distillation
while continuing the esterification of unreacted acid and
transesterification of monoester formed to diester. This process, which
can be applied broadly to esterification of other nitroaromatic acids with
other aliphatic diols, produces high yields of diester without needing
extraneous solvent for processibility and with only water as a by-product.
The 2-methyl-1,3-propanediol-bis-p-aminobenzoate exhibits reactivity and
processing characteristics which make it a suitable drop-in replacement
for MoCA in polyurethane-urea elastomer manufacture.
Inventors:
|
Lassila; Kevin Rodney (Kutztown, PA);
Casey; Jeremiah Patrick (Emmaus, PA)
|
Assignee:
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Air Products and Chemicals, Inc. (Allentown, PA)
|
Appl. No.:
|
646190 |
Filed:
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May 7, 1996 |
Current U.S. Class: |
560/20; 560/50 |
Intern'l Class: |
C07C 205/57; C07C 224/60 |
Field of Search: |
560/20,50
|
References Cited
U.S. Patent Documents
3188302 | Jun., 1965 | Lorenz | 260/77.
|
3428610 | Feb., 1969 | Klebert | 260/75.
|
3736350 | May., 1973 | Meckel et al. | 260/471.
|
3846351 | Nov., 1974 | Huffaker et al. | 260/2.
|
3926923 | Dec., 1975 | Preston | 260/29.
|
3932360 | Jan., 1976 | Cerankowski et al. | 260/77.
|
4218543 | Aug., 1980 | Weber et al. | 521/51.
|
4222955 | Sep., 1980 | Chung et al. | 260/465.
|
4283549 | Aug., 1981 | Holm | 560/50.
|
4476318 | Oct., 1984 | Harada et al. | 560/50.
|
4737527 | Apr., 1988 | Maranci | 523/205.
|
Foreign Patent Documents |
0013956 | Feb., 1980 | EP.
| |
2419322 | Apr., 1973 | DE.
| |
2419322 | Nov., 1980 | DE.
| |
5781445 | Nov., 1980 | JP.
| |
Other References
Baron, et al. "On the Use of Trimethylene Glycol Di-p-aminobenzoate as a
Curing Agent for Polyurethane Elastomers." J Appl. Polym. Sci. 20, pp.
285-286 (1976).
Zey. "Esterification." Kirk-Othmer Encyc. of Chem. Tech. Third Ed.; John
Wiley & Sons; New York. vol. 9, pp. 291-310 (1980).
|
Primary Examiner: Gerstl; Robert
Attorney, Agent or Firm: Rodgers; Mark L.
Parent Case Text
CROSS-REFERENCE TO PARENT APPLICATION
This is a continuation-in-part of application Ser. No. 08/555,199, filed 8,
Nov., 1995, which is a continuation-in-part of application Ser. No.
08/226,916, filed 13 Apr., 1994, both now abandoned the subject matter of
both which is hereby incorporated by reference.
Claims
We claim:
1. 2-methyl-1,3-propanediol-bis-p-aminobenzoate.
2. 2-methyl-1,3-propanediol-bis-p-nitrobenzoate.
Description
FIELD OF THE INVENTION
This invention relates to an aromatic diamine useful as a chain extender in
the manufacture of polyurethane-ureas. In another aspect it relates to an
intermediate p-nitrobenzoate. In yet another aspect it relates to a method
of curing polyurethane compositions with an aromatic diamine and to the
resulting product, and in still another aspect it relates to a process for
making aromatic diamines by esterification of nitroaromatic acids with
diols.
BACKGROUND OF THE INVENTION
Polyurethane-urea elastomers are widely used in industry to fabricate
molded products. These elastomers are typically formed by reacting an
organic polyisocyanate with a compound having a molecular weight between
400 and 10,000 containing at least two Zerewitenoff active hydrogen atoms,
such as a polyhydroxyl compound, and an aromatic diamine chain extending
agent. Alternatively, the chain extender is reacted with an
isocyanate-terminated polyurethane prepolymer. Such prepolymers are well
known in the art. In the molding operation the rate of reaction of the
chain extender, or curative, and the processibility of the reacting
composition is critical. If the reaction proceeds too fast, the
composition will set up or gel before the mold can be completely filled.
On the other hand, if the reaction is too slow, cycle times become too
long and the cost of the operation is excessive. Finding the right
curative for polyurethane-urea elastomers in a particular molding
operation has been the subject of intensive research in this field for
many years.
Three techniques have been used to reduce the reactivity of aromatic
diamines in order to produce polyurethane-urea elastomer molding
formulations with improved processibility. One technique involves
incorporating organic substituents on the aromatic ring to hinder
sterically the amine functionality. Klebert, U.S. Pat. No. 3,428,610
(1969) and Weber et al., U.S. Pat. No. 4,218,543 (1980) describe taking
this approach to the problem, the latter patent also discussing the
importance of reaction rates in the so called "one-shot" reaction
injection molding (RIM) systems where the polyisocyanate, polyhydroxyl
compound and aromatic polyamine are all combined at once rather than using
a prepolymer.
A second technique involves adding an alkyl substituent to the amine
nitrogen which both sterically hinders the amine group and reduces the
number of active hydrogens. An example of this approach is given by
Huffaker et al, U.S. Pat. No. 3,846,351 (1974) with
N,N'-dialkyl-p-phenylenediamine.
The third technique for reducing activity of an aromatic diamine is through
electronic deactivation of the ring. Meckel et al., U.S. Pat. No.
3,736,350 (1973) take this approach by introducing ester and halogen or
alkoxy groups onto the ring.
Lorenz, U.S. Pat. No. 3,188,302 (1965) describes a diamine curative which
takes advantage of both steric hindrance and electronic deactivation to
reduce its reactivity. Representative of such material is
4,4'-methylene-bis(2-chloroaniline) which has been widely used in the art
and is known by its shorthand name "MoCA". MoCA has the additional
advantage of remaining liquid for long periods in the supercooled state
even though it has a relatively high melting point of 130.degree. C. This
enhances its processibility. Unfortunately, as pointed out by Baron et
al., J. Appl. Polym. Sci., 20, pp.285-6 (1976) the Occupational Safety and
Health Administration has placed MoCA on a list of suspected carcinogens
thereby stimulating considerable research for a suitable "drop-in"
replacement. Several candidates are described by Baron et al. and in a
related patent of Cerankowski et al., U.S. Pat. No. 3,932,360 (1976) as
alkylene glycol di-p-aminobenzoates. These curatives are made by reacting
p-nitrobenzoyl chloride with an alkylene or cycloalkylene diol followed by
reduction of the nitro groups to amine. Preferably the diol contains an
odd number of carbons, and more preferably 3 or 5 carbons. All species are
said to possess reasonable supercooling properties with the best candidate
compared with the commercial MoCA being 1,3-propanediol
di-p-aminobenzoate. The curative derived from 1,2-propanediol is said to
have given poor elastomers and was not a good MoCA substitute. Baron et
al. further concluded that the reduced reactivity of these compounds is
attributable to electronic rather than steric effects.
The search to replace MoCA, which for some molding operations is considered
too slow, is illustrated by the '543 patent cited above and by Chung et
al., U.S. Pat. No. 4,222,955 (1980) who describe diamino alkylbenzoates,
alkylbenzonitriles and alkylene bis(amino alkylbenzoates) as polyurethane
curatives which are slower reacting than prior aromatic diamines but
faster than MoCA which frequently requires a catalyst to shorten its
reaction time.
Finding a suitable MoCA replacement in order to meet safety and health
concerns is made more difficult by environmental problems associated with
manufacturing such a replacement. The process described for making the
curatives of the '360 patent, for example, generates hydrochloric acid or
amine hydrochlorides which introduce corrosion, environmental and disposal
problems. In addition, the reaction is conducted in a mutual solvent for
the diol and p-nitrobenzoyl chloride and this requires recovery and
recycling steps for an efficient operation. Japanese Patent Publ. No.
57-81445 (1982) describes making a bis(aminobenzoate) diester by reacting
a dihalide, an amino benzoate and a base, while Harada et al., U.S. Pat.
No. 4,476,318 (1984) disclose making 1,3-propanediol bis(p-aminobenzoate)
by reacting a p-aminobenzoic acid alkali metal salt with dihalogenated
propane in an aprotic polar solvent. Both of these processes produce
halide-containing waste streams and the latter requires solvent separation
and recovery for commercial practicability. An attempt to avoid these
problems as described by Holm, U.S. Pat. No. 4,283,549 (1981) involves
esterification of nitro-benzoic acid and diols in a melt followed by
dissolving the product in a solvent sparingly soluble in water, such as
anisole, and reducing the nitro groups to amine. Holm recognizes that
production of the monoester is undesirable and deals with this problem by
using a stoichiometric excess of the acid in the esterification in order
to drive the reaction to the diester. At the end of the esterification
unreacted acid is converted to its sodium salt by adding water and sodium
carbonate. This salt is soluble in water but sparingly soluble in anisole.
The operation is complex and requires leaching, separation and recovery of
both the organic acid and solvent.
Preston, U.S. Pat. No. 3,926,923, discloses high molecular weight polymers
prepared by reacting at least one aromatic dicarboxylic acid halide and at
least one aromatic diamine having preformed ester units. Included in the
list of diamines useful in the reaction is
2,2-dimethyl-1,3-propane-bis(p-aminobenzoate). This, along with several
other alkylene bis(aminobenzoates) are disclosed in German patent abstract
2419322. Additionally, Maranci, U.S. Pat. No. 4,737,527, discloses heat
curable compositions which contain amine-functional curing agents,
including 2,2-dimethyl-1,3-propanediol-bis-p-aminobenzoate. None of the
diamines disclosed, however, include those of the present invention which
we found exhibit superior results as chain extenders.
It might appear that direct esterification of p-aminobenzoic acid with a
diol would be the ideal solution since the only reaction by-product is
water. Esterification of carboxylic acid and alcohol is an equilibrium
reaction typically driven by carrying out the reaction in an excess of
alcohol or by continuously removing water as it is produced either by
distillation, as an azeotrope, or with a desiccant, as discussed by Zey,
in Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition; John
Wiley & Sons; New York, 1980; Vol. 9, pp 291-310. In diester production
use of excess diol during the reaction is unacceptable since the monoester
would be the primary product. On the other hand, use of excess carboxylic
acid to consume the diol and form more diester creates processing problems
because of the high melting point of the acid and difficulties associated
with removal of excess acid from the desired product. Using stoichiometric
equivalents of organic acid and diol in a melt and forcing the reaction by
removal of water is disclosed in European Patent Appl. No.0 013 956 B1
(1980), but this operation is complicated by the physical form of the
reaction mixture which is difficult to process.
SUMMARY OF THE INVENTION
We have found that a species of organic diamine within the generic
description of diamines found in the '360 patent, cited above, is
surprisingly superior to other known diamines as a chain extender useful
as a drop-in replacement for MoCA in the production of polyurethane-urea
elastomers. This organic diamine is the novel composition
2-methyl-1,3-propanediol-bis-p-aminobenzoate which is not specifically
named or suggested in the prior art. It is made by hydrogenation of its
precursor, 2-methyl-1,3-propanediol-p-nitrobenzoate, which is also a novel
compound useful as the intermediate from which the diamine is formed. This
chain extender is used to react with an organic polyisocyanate and a
compound having a molecular weight between 400 and 10,000 and containing
at least two Zerewitenoff active hydrogen atoms in the production of cast
polyurethane-urea elastomers. The chain extender (or curative) can also be
reacted with a polyurethane prepolymer containing terminal isocyanate
groups to form the urea linkages in the cured product. The processibility
of the diamine and its reactivity rate with isocyanate groups make it
especially attractive for molding large items.
Another aspect of our invention is the preferred method of making the
diamine curative which is the reaction of 2-methyl-1,3-propanediol in
stoichiometric excess with p-nitrobenzoic acid in an esterification which
produces high yields of the desired diester that can then be reduced by
hydrogenation of the nitro groups present to amine. This process can be
applied more broadly to obtain high yields of nitroaromatic diesters,
particularly p-nitrobenzoate diesters, with aliphatic diols having 2 to 12
carbon atoms. The excess diol in the process greatly facilitates
processibility of the reaction mixture so that solvents are not required.
As soon as sufficient diol has reacted with acid to form product including
monoester, thereby becoming relatively nonvolatile, free diol is removed
from the reaction mixture by distillation. Water by-product of the
esterification is also removed in the same manner. "Sufficient diol" is
that amount which is necessary to form the diester with substantially all
of the nitroaromatic acid that is to be reacted. The esterification is
then continued driving the reaction to completion, that is, production of
the diester by both esterification with unreacted acid and by
transesterification of monoester to form diester and diol, which is
removed as it is formed, by reduced pressure if necessary.
This procedure which physically manipulates the presence and absence of
diol during the esterification does not require a solvent, produces no
by-products other than water and provides very high yields of the desired
diester.
DETAILED DESCRIPTION OF THE INVENTION
The composition 2-methyl-1,3-propanediol-bis-p-aminobenzoate is useful as a
chain extender or curative in production of cast polyurethane-urea
elastomers. The formulations cured with this material have exhibited
working times unexpectedly far longer than those cured with the
bis-p-aminobenzoates of the prior art. In addition, this material exhibits
processibility superior to that of previously prepared
bis-p-aminobenzoates in terms of diminished propensity to crystallize from
the melt. Coupled with the desirable physical properties of the elastomer
product, these advantages make the organic diamine of our invention a
prime candidate as a drop-in replacement for MoCA in current industrial
molding operations. Particularly noteworthy is the formation of an
extremely strong yet soft elastomer using commercially available
prepolymers.
The preferred method of making 2-methyl-1,3-propane-bis-p-aminobenzoate
first forms the intermediate p-nitrobenzoate by mixing together under
esterification conditions p-nitrobenzoic acid and a stoichiometric excess
of 2-methyl-1,3-propanediol. The excess diol used is not only more than
that required on an equivalent basis to react with all of the
p-nitrobenzoic acid, but it is also adequate to render the reaction
mixture processible and thereby obviate the need for extraneous solvent.
It has been found that about 1 to 5 mols of diol per mol of acid works
very well in this respect, but somewhat less diol can be used depending
upon the conditions chosen for the reaction. The use of greater amounts of
diol does not adversely affect the reaction but it does reduce reactor
capacity, which is an important consideration for industrial operations.
The esterification reaction is carried out until sufficient diol needed to
convert substantially all of the acid to the diester has reacted to form a
relatively nonvolatile product which includes monoester of the acid and
diol. At this point the reaction mixture typically contains unreacted acid
and diol, monoester, diester and water by-product. This is an easily
processible mixture. Surprisingly, removal of both diol and water from
this mixture does not reduce significantly its processibility but results
in very high yields, on the order of 90%, of the desired diester. The diol
can be removed by distillation, if necessary under subatmospheric
pressures. At the beginning the excess diol greatly enhances the
processibility of the reaction mixture obviating use of an added solvent
which would present recovery and recycle problems. By the time diol is
removed from the reaction mixture, sufficient ester has been formed that
the diol is no longer needed for this function. Although at the outset the
excess diol drives the reaction toward production of the monoester, by
removing diol after the esterification has progressed as described, high
yields of diester are obtained through further esterification of remaining
starting acid and transesterification of monoester with loss of diol. In
this way use of a solvent is avoided, there are no by-products other than
water, the reaction mixture remains readily processible throughout, and
the reaction vessels are used efficiently with high productivity and high
yields of the desired product. The nitro groups on this product are then
hydrogenated to amine groups resulting in the diamine chain extender of
the invention.
This esterification process has broader application than in the production
of 2-methyl-1,3-propanediol-bis-p-aminobenzoate and, in fact, can be used
advantageously to prepare a broad class of dicarboxylate esters of
nitroaromatic acids and aliphatic diols. For example, the nitroaromatic
acid can contain other substituents which do not interfere with the
esterification reaction, such as halogen or alkyl and aryl groups.
Benefits of the process invention can be enjoyed whenever the
nitroaromatic acid has a relatively high melting point making it difficult
to process but the diol with which it is to be reacted is fluid under the
reaction conditions and consequently can serve the function of a process
solvent or reaction medium ensuring efficient contact between reactants
and catalyst. Preferably the nitroaromatic acid is p-nitrobenzoic acid and
the diol is an alkylene or cycloalkylene diol having 2 to 12 carbon atoms.
Such esters can be represented by the structural formula:
##STR1##
wherein G is an alkylene or cycloalkylene group containing 2 to 12
carbons. The nitro groups in the compounds represented by this formula can
be converted to amine groups by hydrogenation using well known techniques
to provide the desired diamines useful as chain extenders in the
production of polyurethane-urea elastomers.
The esterification reaction is carried out in the presence of a catalyst
and at elevated temperatures which are sufficient to complete the reaction
in a reasonable time without decomposition of reactants or products.
Generally the temperatures are in the range of 50.degree. to 200.degree.
C., and preferably above 100.degree. C. The pressure of the reaction is
usually atmospheric, but can be regulated to obtain the desired rate of
distillation removal of water and diol depending upon the particular diol
being used and the reaction temperature.
The catalysts useful in esterification reactions are well known in the art
and include compounds such as mineral acids, for example, sulfuric or
hydrochloric acid, tin salts, organo-titanates, silica gel,
cation-exchange resins, sulfonic acids such as benzenesulfonic acid and
p-toluenesulfonic acid, phosphoric acid, and the like. A preferred
catalyst is titanium (IV) isopropoxide because it has been found that this
catalyst results in fewer by-products. Strong Bronsted acids such as
Amberlyst.RTM.-15 which is a strongly acidic, macroreticular resin
manufactured by Rohm and Haas Co., Nafion.RTM. NR50, a strongly acidic
perfluorinated ion exchange resin manufactured by E.I.du Pont de Nemours &
Co., or methane sulfonic acid are catalysts which produce esters rapidly
but also cause oligomerization of the diol to form an unwanted by-product.
This problem can be addressed by lowering the reaction temperature.
Diols suitable for use in the esterification are aliphatic diols having 2
to 12 carbon atoms. Examples include 1,3-propanediol, 1,5-pentanediol,
2-methyl-1,3-propanediol, 1,3-butanediol, 2,3-butanediol,
1,4-cyclohexanediol, 3-chloro-1,2-propanediol, 1,12-dodecanediol, ethylene
glycol, 2-ethyl-2-methyl-1,3-propanediol, 1,6-hexanediol, 1,8-octanediol,
1-phenyl-1,2-ethanediol, 2,2,4,4,-tetramethyl-1,3-cyclobutanediol, and the
like. The alkylene and cycloalkylene diols are preferred.
Procedures well known in the art for making and molding the
polyurethane-urea elastomers can be used with the diamine chain extenders
provided by our invention. The following examples are presented to
illustrate specific embodiments of our invention and should not by
construed to limit the scope of our invention unduly.
EXAMPLE 1
This example illustrates a preferred procedure for making
2-methyl-1,3-propanediol-bis-p-nitrobenzoate.
A two liter three-necked round-bottomed flask equipped with distillation
takeoff head and nitrogen bubbler was charged with
2-methyl-1,3-propanediol (269.68 g, 2.99 mol) and titanium (IV)
isopropoxide (50.38 g, 0.18 mol). The mixture became cloudy when the
catalyst was added. The contents of the reactor was heated in an oil bath
at 65.degree. C. and 4-nitrobenzoic acid (500.14 g, 2.99 mol) was added
forming a processible slurry. The temperature of the bath was increased to
170.degree. C., and nitrogen was bubbled through the mixture. Water began
to distill from the vessel even before the oil bath had reached the set
temperature. After about 30 minutes, diol was observed in the distillate.
After 5.5 hours, a sample analyzed by gas chromatography showed that the
reaction mixture contained diester and monoester in a weight ratio of
82.2/17.5 with no additional materials other than starting materials
evident. Heating was continued for an additional 15 hours after which the
ratio of diester to monoester had increased to 92.2/7.5. The product was
cooled to 126.degree. C. and 800 mL of mixed xylenes was added. The
resulting solution was filtered hot and cooled to room temperature with
stirring. The crystallized product was collected by vacuum filtration and
dried overnight (80.degree. C., 12 in. Hg) to afford 465 g (80.3% yield)
of product which had a melting point of 122.degree.-125.degree. C. with a
gas chromatographic (GC) assay of 96.8% diester and 3.1% monoester.
Identity of the product was confirmed by .sup.1 H and .sup.13 C NMR and
electron impact and chemical ionization mass spectrometry.
EXAMPLE 2
This example illustrates hydrogenation of p-nitrobenzoate which can be
prepared as described in Example 1 to form the corresponding diamine
useful in polyurethane-ureas as a chain extender.
A two liter autoclave was charged with water-covered Raney.RTM. nickel (25
g), absolute ethanol (950 mL), and
2-methyl-1,3-propanediol-bis-p-nitrobenzoate (303.9 g). The reactor was
sealed and pressure checked with nitrogen, purged three times with
nitrogen and then three times with hydrogen, and pressured to about 50 psi
with hydrogen. The contents of the autoclave was heated to 50.degree. C.
and the hydrogen pressure increased to 500 psi. Heat of reaction increased
the temperature to 80.degree. C. where it was maintained with an internal
cooling coil. After hydrogen uptake ceased (about 50 minutes), the
reaction mixture was cooled to room temperature. The catalyst was removed
by filtration through Celite.RTM., an analytical grade filter agent
composed of diatomaceous earth (Celite Corporation). Absolute ethanol was
used to aid the transfer. The filtrate was concentrated to 900 mL and the
solution allowed to cool to room temperature and placed in an ice water
bath. The resulting solid was collected by vacuum filtration and dried to
constant weight in a vacuum oven to afford 165.1 g of product having a
melting point of 124.degree.-126.degree. C. An additional 55.1 g of crude
product was isolated by evaporation of the solvent after recrystallization
of the solid from the mother liquor. Highly purified product obtained by
recrystallization from absolute ethanol had a melting point of
125.degree.-126.degree. C. Product identity was confirmed by elemental
analysis and by .sup.1 H and .sup.13 C NMR.
EXAMPLE 3
This example illustrates that other diols can be used in the preferred
esterification process of this invention and that the equilibrium of the
reaction can be driven toward diester production by removal of excess diol
under vacuum during the latter part of the reaction.
The diol (60 mmol) was weighed into a 100 mL round-bottomed flask. Catalyst
(10% by weight based on the acid) was added. The flask was fitted with a
distillation head and the contents heated to 60.degree. C. 4-Nitrobenzoic
(60 mmol) was then added and the temperature raised to 170.degree. C.
Water began to condense overhead when the temperature reached 160.degree.
C. indicating that the esterification was under way. At a later point in
the reaction diol removal was initiated while keeping the reaction mixture
fluid and processible. At the end of four hours, the pressure in the flask
was decreased to about 400 torr while continuing to remove diol. After 8
hours, heating was discontinued and the products were analyzed by GC. The
results set forth in Table 1 show that the composition of the product was
about 90% dibenzoate with the rest monobenzoate and unidentified
by-product resulting mainly from diol oligomerization.
TABLE 1
______________________________________
GC Analysis.sup.a
Diol Catalyst Monoester
Diester
Others
______________________________________
1,3-Propanediol
Ti(IV) isopropoxide
4.4 94.9 0.7
1,5-Pentanediol
Ti(IV) isopropoxide
10.0 89.9 0.1
2-Methyl-1,3-
Amberlyst-15 5.7 93.9 0.4
propanediol
______________________________________
.sup.a Flame ionization detector area percent
EXAMPLE 3
Several comparative runs were made to screen catalysts, temperature, and
reaction stoichiometry and to show results obtained when excess diol is
not removed during the course of the reaction.
The catalyst screening runs were made by charging to a 25 mL two-necked
round bottomed flask equipped with a magnetic stirrer, reflux condenser,
and nitrogen inlet 2-methyl-1,3-propanediol (17.7 g, 190 mmol) and
catalyst (0.5 g). This mixture was heated to 60.degree. C. and
4-nitrobenzoic acid (6.35 g, 38 mmol) added after which the mixture was
heated to 175.degree. C. Samples were taken at 1, 4, 6 and 22 hours into
the reaction and analyzed by gas chromatography (GC) using flame
ionization detector area percent for the approximate weight percent of
each component after normalizing out unreacted diol present. Results are
given in Table 2.
TABLE 2
______________________________________
Weight Percent
Catalyst Time Acid Monoester
Diester
Others
______________________________________
Amberlyst-
1 0.0 95.0 0.0 4.8
15 4 0.0 89.4 4.0 6.3
6 0.0 89.0 3.6 7.2
22 0.0 70.8 4.5 24.1
Titanium(IV)
1 3.0 97.0 0.0 0.0
isopropoxide
4 1.1 86.2 11.5 0.0
6 0.9 88.1 10.8 0.0
22 0.9 90.6 8.3 0.0
Nafion 1 0.0 87.5 6.1 6.1
4 0.0 79.2 4.1 16.5
6 0.0 75.4 5.4 18.9
22 0.0 61.2 3.8 34.2
Methane 1 0.0 69.5 7.6 21.6
Sulfonic Acid
4 0.0 64.1 5.2 26.9
6 0.0 61.2 4.4 31.2
22 0.0 48.9 2.4 45.5
______________________________________
The data of Table 2 show that the nitrobenzoic acid reacts rapidly to form
esterification products in the presence of all the catalysts examined.
Titanium (IV) isopropoxide is preferred because it produces the least
amount of oligomerized by-products. With the other catalysts the reaction
is complicated by oligomerization of the diol which accounts for a
significant amount of the "Others" reported in Table 2. It is also
apparent that without removal of excess diol according to the invention,
production of the desired diester is quite low.
In additional runs using Amberlyst-15 catalyst, production of oligomerized
by-products was eliminated by reducing the reaction temperature to
115.degree. C. As a consequence, however, conversion of acid to ester
within the time frame of 1 to 6 hours was significantly lower.
Formation of a processible mixture at the outset of the reaction requires
the use of either solvent or excess diol. Using a solvent has the obvious
disadvantage of introducing recycle or disposal problems to an industrial
operation. As shown by the Table 2 data, excess diol drives the reaction
toward monoester production. In the procedure of the invention, however,
the starting carboxylic acid is reacted sufficiently to trap enough diol
as an esterification product in the reaction mixture to convert
substantially all of the acid to the diester when the reaction is driven
to completion. Once this amount of diol is rendered nonvolatile, the free
diol is removed by distillation and the esterification continues by
reaction of the monoester with unreacted carboxylic acid to liberate water
and by transesterification of monoester to diester with liberation of
diol.
Reaction stoichiometry is important because if too much diol is used at the
beginning of the reaction, reactor productivity is decreased. But if not
enough diol is present, processibility in inadequate. In screening runs
with Amberlyst-15 or titanium (IV) isopropoxide as the catalyst (10% by
weight based on the acid), 4-nitrobenzoic acid and
2-methyl-1,3-propanediol as the reactants in molar ratios of 0.5:1,
1:1,2:1 and 5:1, diol to acid, and a reaction temperature of 173.degree.
C., it was found that the 1:1 molar ratio provided the best balance
between processibility and reactor productivity, although higher ratios
were also practicable. The 0.5:1 ratio, which represents the
stoichiometric requirement for diester production, was not suitable owing
to high viscosity of the reaction mass making it difficult to process.
Although the process of Examples 1 and 2 represents the preferred method of
making 2-methyl-1,3-propanediol-bis-p-aminobenzoate, this novel chain
extender can also be made by other methods, as illustrated by Example 4.
EXAMPLE 4
A three liter three-necked round-bottomed flask equipped with nitrogen
inlet, condenser, thermocouple, overhead stirrer, and addition funnel was
charged with 4-nitrobenzoyl chloride (654.2 g, 3.53 mol) and pyridine (850
mL). A slight exotherm, heating the reaction mixture to 54.degree. C., was
noted. The reaction mixture was heated to 74.degree. C., whereupon all of
the solids in the reaction vessel dissolved. At this point
2-methyl-1,3-propanediol (161.7 g, 1.77 mol) was added to the reaction
solution via the addition funnel over a period of 18 minutes. The color of
the reaction mixture changed from a deep yellow to a light brown and the
exotherm of the reaction heated the solution to 130.degree. C. The
reaction mixture was heated to reflux for five hours, cooled slightly, and
poured onto 2 liters of ice, aiding the transfer by addition of water. The
resulting solid was collected by suction filtration, washed thoroughly
with water, and dried with suction to provide 871.1 g of crude product.
This material was recrystallized from 2500 mL of toluene (hot filtration)
to provide 527.9 g of 2-methyl-1,3-propanediol-bis-p-nitrobenzoate having
a melting point of 125.degree.-126.degree. C. Identity of the product was
confirmed by elemental analysis, by .sup.1 H and .sup.13 C NMR, and by
chemical ionization mass spectrometry.
The nitrobenzoate ester was then converted to the diamine by hydrogenation
as described in Example 2.
The following Examples 5 through 11 compare the novel chain extender of
this invention with closely related commercial chain extenders of the
prior art to demonstrate the surprising and unexpected advantages of the
invention.
EXAMPLE 5
The reactivity of 2-methyl-1,3-propanediol-bis-p-aminobenzoate was compared
to that of Polacure.RTM. 740M which is
1,3-propanediol-bis-p-aminobenzoate, a product of Air Products and
Chemicals, Inc. The procedure for potlife evaluation was that described by
Casey et al., Proceedings of the SPI 28th Annual Technical/Marketing
Conference, pp 218-223 (1984). This test was performed by dissolving the
chain extender (1 eq.) in CAPA.RTM. 200, a polycaprolactone of 274
equivalent weight from Interox (1 eq.), in a stainless steel cup and
preconditioning this mixture at 50.degree. C. for one hour. The
homogeneity of this mixture was ascertained, and Adiprene.RTM. L167, a
toluene diisocyanate (TDI) capped 1000 molecular weight
polytetramethyleneglycol from Uniroyl (2 eq.), thermostatted at 50.degree.
C. was carefully layered on top. The test cup was placed in a
thermostatted block on the test apparatus and a perforated plunger driven
at constant pressure by a reciprocating air motor was activated. Frequency
data for the plunger were stored on a minicomputer and later converted to
relative viscosity. A plot of relative viscosity vs time was produced and
the time required for the mixture to reach a relative viscosity of 5000
(t.sub.5000) was determined. This is a value which provides a concise
reactivity comparison for various chain extenders.
The results of the above evaluations showed that in the initial stages of
the reaction, a period of about 20 to 25 minutes, the viscosities of the
formulations increased at a rate which was essentially independent of the
chain extender used. At this point the viscosity of the composition being
cured with the 1,3-propanediol derivative of the prior art exhibited an
abrupt rise, moving quickly to a t.sub.5000 value of 29 minutes, and a
viscosity above 10,000 within another minute. In sharp contrast, the
viscosity of the composition being cured with the 2-methyl-1,3-propanediol
derivative continued to increase at the same rate to completion of the
reaction, reaching a t.sub.5000 value of 48 minutes, which is very close
to the 42 minute potlife of MoCA. This result indicates that the chain
extender of this invention could be used as a replacement for MoCA in
industrial production with a minimum of operational adjustments.
The observed difference in cure profiles of these two aminobenzoate chain
extenders was quite unexpected because the reactivity, and hence the
potlife, of the aminobenzoate of the prior art is said to be mainly a
function of the electronic nature of the molecule (see Baron, et al.
supra). Merely placing a methyl substituent symmetrically on the propylene
group linking the p-aminobenzoates in the molecule would not be expected
to impact so heavily the cure profile of the chain extender. In addition
to enhancing its value as a replacement for MoCA, this extended cure
profile for the product of the invention enables longer working times in
large molding operations and provides greater flexibility in such
industrial applications.
EXAMPLE 6
This example compares the melt stability of the two p-aminobenzoates
compared in Example 5. Cast elastomers prepared with solid chain extenders
are commonly made by adding the molten chain extender to an isocyanate
prepolymer. A key parameter for determining the processibility of the
chain extender is the temperature at which the molten chain extender
begins to crystallize, for when this occurs, processing is considerably
complicated. Thus, a chain extender which remains liquid, even when
supercooled, is more processible.
Samples of 1,3-propanediol-bis-p-aminobenzoate and
2-methyl-1,3-propanediol-bis-p-aminobenzoate were each placed in capillary
tubes and heated to 135.degree. C. in a Thomas-Hoover oil immersion
melting point apparatus. At this point, heating was discontinued, and the
time was recorded. After 5 minutes, the oil temperature had reached
90.degree. C. and 1,3-propanediol derivative was beginning to form
crystals while the 2-methyl-1,3-propanediol derivative remained molten.
After 6 minutes, the temperature had reached 81.degree. C. and all of the
1,3-propanediol-linked material had crystallized but the
2-methyl-1,3-propanediol-linked compound had not. After 25 minutes, the
temperature had decreased to 40.degree. C. and still no transition to a
solid had occurred for the composition of the invention.
Thus, it has been shown that 2-methyl-1,3-propanediol-bis-p-aminobenzoate
remains substantially liquid even when cooled over 80.degree. C. below its
normal melting point. Supercooling properties was one of the advantages
pointed out by the '360 patent for the compositions disclosed, and since
the 1,3-propanediol-linked derivative was the compound most preferred, it
was not expected that the 2-methyl-1,3-propanediol-linked derivative of
this invention would be so markedly superior in this respect. The chain
extender of this invention, therefore, exhibits surprisingly enhanced
processibility.
EXAMPLE 7
Accurately weighed samples of 1,3-propanediol-bis-p-aminobenzoate and
2-methyl-1,3-propanediol-bis-p-aminobenzoate were placed in pans for
differential scanning calorimetry. The pans were placed in the DSC
apparatus and the temperature was adjusted to 100.degree. C. The
temperature was then increased at a rate of 2.degree. C. per minute to
150.degree. C. The temperature was then decreased at a rate of 1.degree.
C. per minute to 25.degree. C., and then increased again to 150.degree. C.
at a rate of 2.degree. C. per minute.
Both compounds exhibited a melting endotherm upon initial heating. During
the first cooling 1,3-propanediol-bis-p-aminobenzoate exhibited a
crystallization exotherm at 90.degree. C., but no crystallization exotherm
was observed for the 2-methyl-1,3-propanediol-bis-p-aminobenzoate even on
cooling to room temperature. Both samples were then reheated to
150.degree. C. at a rate of 2.degree. C. per minute. The 1,3-propanediol
derivative exhibited a second melting endotherm at 128.degree. C. No
endotherm was observed during the second heating of the sample of the
2-methyl-1,3-propanediol derivative, confirming that none of the sample
had recrystallized from the melt.
These results show that 2-methyl-1,3-propanediol-bis-p-aminobenzoate
produces a much more stable melt than the 1,3-propanediol derivative of
the prior art. This property for the diamine of the invention considerably
simplifies processing as a polyurethane-urea elastomer chain extender
because once melted it remains molten at temperatures below its melting
point for a considerable period of time.
EXAMPLE 8
Often in cast polyurethane-urea manufacture, a solid diamine chain extender
is dissolved in a polyether or polyester diol to simplify handling and
enhance processibility. The dissolution process is typically performed by
heating a mixture of the diamine chain extender and the polyol until it
becomes homogeneous. If, when this solution is cooled, the diamine chain
extender begins to crystallize or precipitate, casting is complicated.
Further, since the crystallized material is unlikely to react efficiently,
an article of poor quality will probably be formed.
In order to compare the behavior of the composition of the invention with
that of the prior art when dissolved in polyol, two 10 mL vials were
charged with PolyTHF650, a polytetramethylene glycol of 650 molecular
weight manufactured by BASF (1.08 g) and either
1,3-propanediol-bis-p-aminobenzoate (1.05 g) or
2-methyl-1,3-propanediol-bis-p-aminobenzoate (1.09 g). Both mixtures were
heated until they were homogeneous and then allowed to cool under ambient
conditions. The temperature was monitored by means of a thermocouple
immersed in each mixture. The results are summarized in Table 3.
TABLE 3
______________________________________
Linking Group
Time (min)
Temp (.degree.C.)
Observations
______________________________________
1,3-propanediol
0 160 Homogeneous solution
7 41 Some solid present
16 28 Additional solid
34 26 Still more solid
91 26 Abundant crystals
180 Ambient Complete crystallization
2-methyl-1,3-
0 155 Homogeneous
propanediol
15 29 Homogeneous
32 27 Homogeneous
89 26 Homogeneous
1 day Ambient Minor crystal formation
4 days Ambient Complete crystallization
______________________________________
The above reported results showed that the diamine chain extender of this
invention exhibited a much lower rate of recrystallization than the prior
art material, thereby having superior processibility.
EXAMPLE 9
The minimum temperature at which crystal formation can occur in a mixture
of polyol and diamine chain extender is also important since this
determines an appropriate storage temperature and provides a temperature
above which processing problems are not likely to occur. The diamine chain
extenders and the PolyTHF650 used in Example 8 were mixed in the same
ratios and heated in an oil bath until the chain extenders dissolved
(130.degree. C.). The temperature of the bath was successively decreased
in 5.degree. C. increments and allowed to stand at that temperature for a
period of 30 minutes. Once the temperature at which crystal formation was
evident had been determined, the samples were slowly heated to the minimum
temperature at which they again became homogeneous. The temperature was
oscillated between these crystallization-solution regimes until the
minimum temperature at which the samples were homogeneous and the maximum
temperature at which crystals could be observed was determined.
For the 1,3-propanediol-bis-p-aminobenzoate, at 123.degree. C. crystals
were observed after one hour while at 124.degree. C. the solution was
homogeneous.
For the 2-methyl-1,3-propanediol-bis-p-aminobenzoate, at 100.degree. C.
crystals were observed after about 24 hours while at 105.degree. C. the
solution was homogeneous. These tests again showed that the diamine chain
extender of the invention exhibits superior processibility.
EXAMPLE 10
This example compares physical properties of polyurethane-urea elastomers
using the chain extender of the invention with elastomers using commercial
chain extenders, including MoCA. Plaques were cast by mixing neat molten
chain extender (1.0 equivalent) with prepolymer (1.05 equivalents) at
70.degree. C. and placing the mixture in a mold having inside dimensions
of 6 in..times.6 in. by 1/8 in. after the mold had been preheated to
100.degree. C. The material was then pressed at 100.degree. C. and about
15 tons and cured in the mold until the part had developed sufficient
mechanical strength to be removed (about 1 hour). The elastomer was then
postcured at 100.degree. C. for a period so that the total cure time was
16 hours. Defect-free pieces were cut from these plaques and their
properties were measured according to ASTM procedures D-412-83
(Microtensile), D-624-81 (Die C Tear), and D-2240-81 (Durometer Hardness).
The results are given for each elastomer in Table 4. Elastomers made from
three different prepolymers were evaluated, and each prepolymer was cured
with 2-methyl-1,3-propanediol-bis-p-aminobenzoate and at least one
commercial chain extender. Group A elastomers were made with Adiprene.RTM.
L167, group B with Airthane.RTM. PET 95A (a TDI-capped 1000 molecular
weight polytetramethylene glycol containing less than 0.1% free TDI,
manufactured by Air Products and Chemicals, Inc.), and group C were made
with Airthane.RTM. PET 70D (a TDI-capped 650 molecular weight
polytetramethylene glycol containing less than 0.1% free TDI, manufactured
by Air Products also). In Table 4, the elastomers cured with the diamine
of the invention are indicated by (1), those cured with
1,3-propanediol-bis-p-aminobenzoate (Polacure.RTM. 740M) by (2), those
cured with MoCA by (3), and an elastomer cured with Ethacure.RTM. 300 (a
chain extending agent of 80% 3,5-di(methylthio)-2,4-toluenediamine and 20%
3,5-di(methylthio)-2,6-toluenediamine obtained from Ethyl Corporation)
indicated by (4).
TABLE 4
______________________________________
Hardness Tensile Strength (PSI)
Elongation
Tear
Elastomer
Sh.A Sh.D 100% 200% 300% Break
(at break)
Die C
______________________________________
A (1) 80 35 310 500 760 4190 660% 160
A (2) 81 41 270 400 590 2320 750% 90
B (1) 90 42 640 1200 2400 7490 520% 290
B (2) 97 49 1770 2330 3080 5110 480% 650
B (3) 97 48 2030 2690 3800 4510 360% 600
B (4) 95 55 2610 3520 4920 5250 330% 460
C (1) 100 70 3610 5130 na 6490 280% 980
C (3) 97 67 4990 6080 na 6560 240% 1080
______________________________________
The data of Table 4 show that when either Adiprene.RTM. L167 or
Airthane.RTM. PET 95A was used as the prepolymer, the curative of the
invention provided elastomers (A1 and B1 ) which were softer but stronger
at break than the elastomers obtained with these prepolymers cured with
1,3-propanediol-bis-p-aminobenzoate (A2 and B2) or the other chain
extenders (B3 and B4). This combination of properties is unusual and
unexpected. Such a combination of high strength and softness is of
particular value in articles like tires and paper rollers.
When Airthane.RTM. 70D was the prepolymer, the chain extender of the
invention produced an elastomer with properties comparable to those
obtained with MoCA. The excellent physical properties coupled with the
fact that the processibility was similar to that of MoCA means that
2-methyl-1,3-propanediol-bis-p-aminobenzoate is more nearly a drop-in
replacement for MoCA than other prior art chain extenders. This result is
quite surprising when one considers that
1,3-propanediol-bis-p-aminobenzoate (Polacure.RTM.) was introduced as an
alternative to 1,4-methylene-bis-ortho-chloroaniline (MoCA), the industry
standard chain extender for polyurethane-urea cast elastomers. The
potential for the composition of this invention, meanwhile, went
undetected and unappreciated.
EXAMPLE 11
Runs were carried out to compare the performance of
2,2-Dimethyl-1,3-propanediol-bis-p-aminobenzoate as a chain extender with
that of the compound of the present invention.
Preparation of 2,2-Dimethyl-1,3-propanediol-bis-p-nitrobenzoate. The
procedure used was that outlined in U.S. Pat. No. 3,932,360 ›Cerankowski,
et al.!. To a 500 mL three-necked round-bottomed flask equipped with
overhead stirrer, heating mantle, condenser, and nitrogen inlet were added
4-nitrobenzoyl chloride (74.22 g, 0.40 moles),
2,2-dimethyl-1,3-propanediol (20.83 g, 0.2 mol) and pyridine (100 mL). The
reaction was heated to reflux with stirring for 5 hours, and then cooled
to room temperature. The resulting slurry was added to 1500 mL of water
and stirred for ca. 30 minutes. The solid product was collected by suction
filtration, and then stirred overnight in 1 L of water. The product was
again collected by filtration, washed with 2.times.200 mL of water, air
dried in the filter for 1 h, then dried in a vacuum oven overnight
(91.degree. C., 13.2 in Hg, slight N.sub.2 purge through oven) to afford
75.96 g of pale tan powder (mp 147.9.degree.-148.3.degree. C.) identified
as 2,2-dimethyl-1,3-propanediol-bis-p-nitrobenzoate by nuclear magnetic
resonance (NMR) spectroscopy.
Preparation of 2,2-Dimethyl-1,3-propanediol-bis-p-aminobenzoate. The
diamine was prepared by hydrogenation (55.degree. C. and 550 psig) of the
dinitro compound (10.0 g) in the presence of 10% Pd-C catalyst (0.5 g) in
50 mL of tetrahydrofuran. Gas uptake stopped about 20 minutes after
reaching the reaction temperature. The reaction mixture was cooled and the
catalyst was removed by filtration. The solvent was removed and the
residue was recrystallized from ca. 75 mL of absolute ethanol to afford
6.0 g of off-white solid (mp 162.degree.-164.degree. C.). The product was
identified as 2,2-dimethyl-1,3-propanediol-bis-p-aminobenzoate by nuclear
magnetic resonance spectroscopy.
Melt Stability of 2,2-Dimethyl-1,3-propanediol-bis-p-aminobenzoate. The
procedure of Example 6 above was repeated using the subject material. When
a sample of 2,2-dimethyl-1,3-propanediol-bis-p-aminobenzoate was heated,
the material did not melt until the temperature reached
162.degree.-164.degree. C. When heating was discontinued, the temperature
dropped to 113.degree. C. in 4 minutes, and the material began to
crystallize. After about an additional minute, the temperature had reached
107.degree. C., and the material had completely crystallized. The
transition from the melt to the solid occurred at a temperature of nearly
30.degree. C. higher than the corresponding transition for
1,3-propanediol-bis-p-aminobenzoate, the compound identified as preferred
in the prior art. Crystallization from the melt at this temperature makes
this material unsuitable for use as a chain extender in cast
polyurethane-ureas.
Differential Scanning Calorimetry of
2,2-Dimethyl-1,3-propanediol-bis-p-aminobenzoate. A procedure similar to
that of Example 7 above was performed. Note, however, that because of the
high melting point of this compound, the sample had to be heated to
180.degree. C., rather than the 150.degree. C. used in the other
experiments.
An accurately-weighed sample of
2,2-dimethyl-1,3-propanediol-bis-p-aminobenzoate was placed in a pan for
differential scanning calorimetry. The pan was placed on the DSC apparatus
and the temperature was adjusted to 100.degree. C. The temperature was
then increased at a rate of 2.degree. C. per minute to 180.degree. C. The
temperature was then decreased at a rate of 1.degree. C. per minute to
25.degree. C., and then increased again to 180.degree. C. at a rate of
2.degree. C. per minute.
The compound exhibited a melting endotherm of 116 J/g during the first
heating at a temperature of 165.degree. C. No clear crystallization
exotherm was observed upon cooling. However, when the sample was reheated,
two crystallization exotherms (70 J/g at about 73.degree. C. and 1 J/g at
about 82.degree. C.) were observed. Continued heating resulted in the
observation of a crystallization endotherm (116 J/g) at 163.degree. C.
The magnitude of the melting endotherm was 116 J/g for both the first and
second heatings, indicating that the product had completely recrystallized
prior to the second melting. Further, the sum of the heat released during
the crystallization exotherms (70 J/g) was significantly lower than the
melting endotherms, indicating that the material had partially
crystallized when cooled to room temperature.
Use of this product as a chain extender in polyurethane-urea cast elastomer
manufacture would be difficult because of the high temperatures required
to reach a molten state. Even if equipment capable of heating this
material to the melt were available, processing would be complicated by
the fact that a significant amount of crystallization occurs during
cooling. This behavior is to be contrasted to that of the diamine of this
invention which melts at a lower temperature and exhibits no
crystallization from the melt when cooled to room temperature.
Crystallization from Polyol. The procedure of Example 8 above was repeated
with 2,2-dimethyl-1,3-propanediol-bis-p-aminobenzoate (1.01 g) and
PolyTHF650 (1.01 g). The following data were obtained:
______________________________________
Linking Group
Time (min)
Temp (.degree.C.)
Observations
______________________________________
2,2-dimethyl-1,3-
0 160 Homogeneous
propanediol
2 134 Solids visible
4 95 Complete crystallization
______________________________________
The above results clearly show that the performance of this chain extender
is inferior to the 2-methyl-1,3-propanediol-bis-p-aminobenzoate of the
present invention.
Other advantages and features of our invention will be apparent to those
skilled in the art from the foregoing disclosure without departing from
the spirit or scope of our invention.
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